Studies were undertaken to assess the Brucella infection status of a feral swine herd which was previously known to be enzootically infected with Brucella and to assess potential Brucella vaccines in both domestic and feral swine populations. Eighty feral swine were trapped in South Carolina. The Brucella culture positivity rate was 77.5%. Both B. suis and B. abortus were cultured from infected feral swine. Of the B. abortus isolates, B. abortus biovar 1 field isolates and B. abortus S19 were identified. The serological positivity rate was 48.8%, and the sensitivity of individual serological assays ranged from 13.1%-44.6%. Histological lesions compatible with Brucella infection were also noted. The natural, rough mutant vaccine B. abortus RB51 was evaluated in domestic and feral swine. Domestic swine failed to develop significant cell-mediated or humoral immune responses following parenteral vaccination with 2.0 X 1010 CFU. Also, vaccination with RB51 provided no protection from challenge with virulent B. suis in gravid gilts. RB51 failed to reduce the seroconversion rate or culture recovery rate of vaccinated feral swine. A natural rough mutant of B. suis, B. suis 353-1, was evaluated in domestic swine. B. suis 353-1 induced significant cell-mediated responses in domestic swine parenterally and orally vaccinated with a live preparation as well as domestic swine parenterally vaccinated with a heat-killed, adjuvanted preparation. The vaccine was nonpathogenic and was not shed from animals vaccinated with live preparations. The rough phenotype was stable for the duration of the studies. Domestic swine vaccinated with all preparations of 353-1 had some level of protection from challenge with virulent B. suis.